In electrical discharge machining (EDM), a metal workpiece immersed in a bath of an ionizable dielectric is machined to a desired shape by electrical erosion. That is, an electrode of the required configuration is lowered into the bath, in proximity to the workpiece, and a series of electrical current discharges are generated between the electrode and the workpiece. Each spark or arc discharge between the electrode and the workpiece results in limited erosion (cutting) of the workpiece. With continuing movement of the electrode toward the workpiece, the workpiece is ultimately shaped to the configuration of the electrode. This kind of machining is particularly useful in the cutting of irregular cavities or external configurations, though the technique can be employed for relatively simple and symmetrical shapes as well.
Although electrical discharge machining follows the shape of the electrode quite precisely, the cavity or other surface cut into or on the workpiece does not conform exactly to the electrode dimensions, differing therefrom by the length of the arc gap that must be maintained between the electrode and the workpiece during machining. On the other hand, for maximum efficiency in an EDM operation, the erosion rate is preferably not maintained constant. For initial rough shaping, a high working speed is utilized, with arcs of maximum intensity that remove relatively large amounts of metal. For surface finishing, arcs of lesser energy are employed, over a shorter arc path.
In a conventional EDM process, the requirement for a change in the length of the arc gap has often necessitated the use of a series of electrodes, all of similar configuration but having dimensions adapted to the particular machine speed at which each was to be used. The expense and difficulty of providing a series of electrodes of this kind is effectively eliminated by the use of orbital electrode supports, particularly as described in Braudeau et al U.S. Pat. No. 3,433,919 issued Mar. 18, 1969. With this technique, the electrode is moved horizontally during the EDM process, particularly during that portion of the machining operation required to provide a final finished surface, in what is usually referred to as an orbital path. The horizontal motion of the electrode may follow a circular path; however, depending upon the configuration of the cut being made in the workpiece and the finish requirements for the machining operation, the transverse or horizontal motion of the electrode may be along an oval path or an irregular path, or may even approach linear reciprocation. Any and all of these types of motion are intended to be included in any reference to "orbital motion" or to an "orbital path" in this specification.
In known orbital tool supports, the apparatus for generating the horizontal orbiting motion of the electrodes is usually mounted in direct centered relation on the vertically movable platen or tool support head of a conventional EDM machine, with the electrode supported directly from the orbiter. Machines of this kind are described in the aforementioned Braudeau patent, in Mayer et al U.S. Pat. No. 3,322,929 issued May 30, 1967, and in Bentley et al U.S. Pat. No. 3,135,852 issued June 2, 1964. In other known arrangements, on the other hand, the orbiting head has been mounted on a vertical axis displaced from the machine axis; see, for example, Furze et al U.S. Pat. No. 3,539,754 issued Nov. 10, 1970 and Weber U.S. Pat. No. 3,809,852 issued May 7, 1974.
A substantial problem presented in connection with either type of EDM orbiting head, as referred to above, is a lack of dynamic stability, particularly in jobs requiring the use of large electrodes or multiple electrodes. The lack of stability arises from several sources, including the weight of the electrode structure itself, if the electrode is of irregular or eccentric configuration and hence causes eccentric loading of the orbiting tool support structure. A more important source of eccentric loading contributing to dynamic stability, however, arises from the reaction forces occurring in the course of the machining operations. These forces are substantial and they are inherently eccentric, due to the orbital movement of the electrode, which concentrates arcing at successive different horizontal locations in the course of the machining operation. The frequent result of this lack of dynamic stability is difficulty in controlling machining stability and maintaining close tolerances when an orbiting head is used as the electrode support system, particularly on jobs requiring the use of large electrodes or multiple electrodes. The same general considerations are applicable in other machine tools in which orbiting heads may be used, as in electro chemical machining (ECM) systems and in milling machines or other mechanical-cutting machine tools.